Re: Dots of gum?

From: Etienne Garbaux ^lt;[email protected]>
Date: 02/13/05-12:24:14 AM Z
Message-id: <p05210601be348a195482@[192.168.1.100]>

Katharine wrote:

> Another thing I've been thinking about lately is this idea that gum
> sticks better * * * if one uses a half-tone or stochastic dot negative.
>
> But I realized that something isn't making sense to me about this. The
> argument has always run this way: somehow these discrete dots of gum
> that are laid down by halftone or stochastic dot negatives stick to the
> substrate better than gum that's not laid down this way. But here's the
> problem I have with this argument: since it's the negative that is made
> of discrete dots, what prints on the gum isn't the discrete dots. What
> prints is the space around the dots. So I don't get this argument; could
> someone make this work for me?

Both Mark and Judy have identified the mechanism, at least as my
understanding has it to a first approximation.

Printing gum is in some ways very much like making dye transfer matrices or
carbon prints. In all three, one exposes a layer of soluble gel to harden
it and make it insoluble. In all three, there is a depth effect to the
exposure -- the surface closest to the exposing light hardens first, and
the hardening proceeds through the thickness of the gel layer according to
exposure. The remaining soluble gel is then washed away, leaving a
representation of the tonal values in the amount of gel remaining at any
point on the image.

In DT matrix preparation and carbon printing, the process is arranged so
that the maximally-hardened surface of the gel is bonded to a support
before developing. In carbon printing, this is usually done by
transferring the exposed pigmented gel to another support before
development. DT matrix film is exposed through the base, so the
maximally-hardened layer is already next to the support.

Gum, on the other hand, is developed with its hardened layer away from the
support and soluble gel next to the support. If one tries developing
carbon tissue this way, or exposing DT matrix film from the emulsion side,
the whole image washes away, except for a few spots where it was exposed
sufficiently to harden the gel all the way through to the support. Gum,
therefore, depends on the tooth of the support -- having substrate fibers
more or less throughout the depth of the coating -- so that the hardened
gel has something to grab onto to prevent being washed away. Thus, it
tends to work best with rather woolly papers, not as well with hot press,
and even less well on toothless substrates like glass. This feature of gum
is widely credited with making it more difficult to get a wide and smooth
tonal range with gum than it is with other processes, particularly on
smooth paper -- which is believable. [Obviously, some practitioners are
able to get a very nice tonal range with gum -- it is not my intention to
start a "gum has limited tonal range" war, and shame on anyone who takes
this discussion in that direction.]

The advantage, if there is one, of a halftone image is that wherever the
gum is exposed, it is more or less exposed all the way through to the
support. Even in the lighter midtones, where a continuous-tone negative
would leave most of the area with a thick layer of soluble gum between the
support and the hardened gum, the halftone-exposed gum should produce an
approximately correct (if grainy) tone. The gum you are washing away isn't
underneath the gum you want to stay, it's beside it.

If I wanted a gum image on glass, I think I'd coat the glass with something
that hardened gum sticks to very well. Perhaps a thin layer of hardened,
unpigmented gum, using an intermediary subbing material between the glass
and the hardened gum layer if necessary. At this point, one would have the
same two choices we have already seen: coat pigmented gum onto the hardened
gum layer and expose through the glass, or make a pigmented gum "tissue,"
expose it normally, then adhere it to the hardened layer on the glass with
cool water, let it dry, and then develop it in warm water. The former
would make it difficult to do multiple layers. The latter might give
problems with the continuing post-exposure hardening effect of dichromates,
although perhaps a good rinse could be incorporated into the cool water
adhering process to stop this.

On the topic of chemical tooth: chemical tooth is a molecular effect.
Very simply, good subs are molecules which have one end with an affinity
for the support and the other with an affinity for whatever you want to
adhere to the support. This can work excellently, and solves some thorny
industrial problems. However, because the chemical tooth is only a
molecule deep, I do not believe it could "reach through" the thickness of a
gum layer to adhere the hardened gum away from the support if there were
still soluble gum in between. So it might be a boon in conjunction with
either of my suggestions above, but probably won't be a viable solution for
any "support one surface and expose the other one" system.

Best regards,

etienne
Received on Sun Feb 13 00:24:32 2005

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